1. Field of the Invention
The present invention relates a roller screw which comprises a screw shaft, a nut member,and a plurality of rollers interposed between the screw shaft and nut member.
2. Description of the elated Art
Conventionally, the e is known a ball screw in which a plurality of balls are interposed between a screw shaft and a nut member. Now, FIG. 5 shows an axial-direction section of a ball screw of this type. In case where a screw shaft 1 is rotated with respect to a nut member 2, the number member 2 is allowed to reciprocate in the axial direction of the screw shaft 1. Balls 3 are all wed to roll between the screw shaft 1 and nut member 2 along the outer periphery of the screw shaft 1. By the way, the ball screw, in some cases, can be given an axial-direction load (1) or an axial-direction load (2) as an external load. The axial-direction load (1) is an external load which occurs when the nut member 2 moves forward, whereas the axial-direction load (2) is an external load which occurs when the number 2 moves backward. These two axial-direction loads (1) and (2) are directed in the mutually opposite directions. As shown in FIG. 5, the right and left half sections of each ball 3 are symmetric in the axial-direction section thereof and, therefore, either of the left and right surface 3a or 3b of the ball 2 is able t receive the load. That is, the ball screw is able to receive both of the axial-direction load (1) and (2) which occur in he reciprocating motion thereof.
In some of the ball crews, there can be a case where there is a great difference in size between the two axial-direction loads (1) and (2). For example, in case where the ball screw is used in an injection molding machine, when fluidized resin is pushed into a metal mod using a push-in device, dozens of tons of axial-direction load is applied to the ball screw, whereas, when the push-in device itself is moved back, only a slight axial-direction load is a plied to the ball screw.
Conventionally, as a all screw suitable for the case where the above-mentioned axial-direction loads occurring in the forward and backward are different in size, as shown in FIG. 5, there is known a ball crew (see JP-A-11-257455) in which not only a portion of a 5 spiral-shaped load rolling groove 2a formed in the nut member 2 is shifted with respect to a spiral-shaped load rolling groove la formed in the screw shaft 1 but also the number of effective windings of the load rolling groove 2a for receiving a high load is set larger than the number of effective windings of the load rolling groove 2a for receiving a low load. According to this ball screw, a contact angle line 4, which is obtained by connecting together two contact points, that is, a contact point between the ball 3 and the ball rolling groove 1a formed in the screw shaft 1 and a contact point between the ball 3 and the ball rolling groove 2a formed in the nut member 2, can be incline to thereby receive the two different forward and backward axial-direction loads (1) and (2) applied to the ball screw during the reciprocating motion thereof.
Also, in case of a roller screw in which rollers are interposed between a nut member and as screw shaft, the nut member reciprocates with respect to the screw haft in the axial direction thereof; and, therefore, the roll r screw is requested to be able to receive not only an axial-direction load occurring in the forward motion thereof but also a axial-direction load occurring in the backward motion thereof. However, each of the rollers, unlike the ball, is not formed symmetric (that is, it is not spherical in shape). There ore, in the case of the roller screw, unlike the ball screw, the direction of the axial-direction load that can be received by the roller screw is limited to one direction; and thus, the roller screw is not be able to receive both of the axial direction loads that respectively occur in the forward and backward motion of the roller screw. Even in case where, as di closed in JP-A-257455, a portion of a load rolling groove formed in the nut member is shifted with respect to a roller rolling groove formed in the screw shaft, the roller screw can not receive both of the axial-direction loads respectively occurring in the forward and backward motion of the roller screw.
As a roller screw which is structured such that it is able to receive both of the axial-direction loads respectively occurring in the forward and backward motion thereof, there is known a roller screw which is disclosed in JP-A-210858. FIG. 6 shows an axial-direction section view of this roller screw, and FIG. 7 shows rollers which are arranged and stored in a roller rolling groove formed in the roller screw shown in FIG. 6. As shown in FIGS. 6 and 7, between a V-shaped load rolling groove formed in a nut member 5 and a V-shaped load rolling groove formed in a crew shaft 6, there are cross arranged rollers 7; that is, the mutually adjoining rollers 7 cross each other. More specifically, the roller 7, which rolls between the wall surface 5a of the load rolling groove and the wall surface 6a of the load rolling groove, and the roller 7, which rolls between the wall surface 6a of the load rolling groove and the wall surface 6b of he load rolling groove, are arranged alternately.
However, in the roller crew of this type, since the mutually adjoining rollers 7 cross each other, when the axial-direction load (1) or the axial-direction load (2) is applied to the roller screw, the rolling roller and the roller 7, which does not roll but slides, are arranged alternately. In this alternate arrangement of rollers, there is a fear that the rolling motion of the rollers 7 or the pick-up operation of the rollers 7 can be interfered. Also, to be able to pick up the mutually crossing rollers 7 with a single return pipe, the section shape of a no-load return passage within the return pipe and the side surface shapes of the rollers must be square, which limits the shape of the return pipe or the shapes of the rollers that can be used. Further, since the number of rollers 7 for receiving the axial-direction load (1) occurring in the forward motion of the roller screw is set equal to that of rollers 7 for receiving the axial-direction load (2) occurring in the backward motion, when the axial-direction load (1) in the forward motion and the axial-direction load (2) in the backward motion are different in size, a roller screw must be selected according to the higher one of the two loads, which makes it inevitable that the whole length of the nut member is long.
The present invention aims at eliminating the drawbacks found in the above-mentioned conventional roller screws. Accordingly, it is an object of the invention to provide a roller screw and a method for arranging rollers in a roller screw, which not only can receive both of the forward- and backward-motion axial direction external loads but also can prevent the circulatory motion of rollers from being interfered.
Incidentally, in the following description, for the purpose of easy understanding of the invention, the reference numerals of the accompanying drawings are given in parentheses. However, it should be noted here that the invention is not limited to embodiments illustrated herein.
In attaining the above object, according to a first aspect of the invention, a plurality of rollers to be interposed between a nut member and a screw shaft are composed of a forward-motion load roller group the rollers of which are arranged in such a manner that the center lines thereof are set at a given angle with respect to the center line of the screw shaft, and a backward-motion load roller group the rollers of which are arranged in such a manner that the center lines thereof are set at an angle, which is different from the above-mentioned given angle, with respect of the center line of the screw shaft; and, the forward-motion load roller group and backward-motion load roller group are disposed such that they are separated from each other.
That is, according to the first aspect of the invention, the above-mentioned problems can be solved by providing a roller screw which comprises: a screw shaft (12) having a spiral-shaped roller rolling groove (11) formed in the outer peripheral surface thereof; a nut member (14) having a spiral-shaped load rolling groove (13) so formed in the inner peripheral surface thereof as to correspond to spiral-shaped roller rolling groove (11), and a plurality of rollers (16) to be arranged and stored between the roller rolling groove (11) and load rolling groove (13), wherein, as the screw shaft (12) is rotated with respect to the nut member (14), the nut member (14) is allowed to reciprocate with respect to the screw shaft (12), characterized in that the plurality of rollers (16) are composed of a forward-motion load roller group (17) the rollers (16) of which are arranged in such a manner that to center lines thereof are set at a given angle (xcex1) with respect to the center line of the screw shaft (12), and a backward-motion load roller group (18) the rollers (16) of which are arranged in such a manner that the center lines thereof are set at an angle (xcex2), which is different from the above-mentioned given angle, with respect to the center line of the screw shaft (12 ; and, the forward-motion load roller group (17) and backward-motion load roller group (18) are disposed such that they are separated from each other.
According to the first aspect of the invention, since the forward-motion load roller group (17) receives a forward-motion axial-direction load ((1)) applied to the roller screw during the reciprocating motion of the roller screw and backward-motion load roller group (18) receives a backward-motion axial-direction load ((2)) applied to the roller screw during the reciprocating motion of the roller screw, the roller screw is able to receive both of the forward- and backward-motion axial-direction loads. Also, not only because the forward-motion load roller group (17) and backward-motion load roller group (18) are separated from each other but also because the mutually adjoining rollers (16) of the forward-motion load roller group (17) and backward-motion load roller group (18) face in the same direction, there is eliminated a fear that the rolling motion of the rollers (16) with in the load rolling passage (15) as well as the picking-up operation of the rollers (16) by the return pipes (20, 21) can be interfered, thereby allowing the rollers (16) to circulate smoothly.
Also, according to second aspect of the invention, in a roller screw according to the first aspect of the invention, the winding numbers of the forward-motion load roller group (17) and backward-motion load roller group (18) are so set as to be different from each other according to the ratio of axial-direction loads applied to the roller screw during the reciprocating motion there of. Here, the ratio of loads means a ratio of the size of an axial-direction external load ((1)) occurring in the forward motion of the roller screw to the size of an axial-direction external load ((2)) occurring in the backward motion of the roller screw.
According to the second aspect of the invention, in accordance with the ratio of the axial-direction loads applied to the roller screw, the wending number of the roller group which receives a high load can be increased and the winding number of the roller group which receives a low load can be decreased, which makes it possible to minimize the whole length of the nut member (14).
According to a third aspect of the invention, in solving the above-mentioned problems, there is provided a roller screw which comprises: a screw shaft (42) having a spiral-shaped roller rolling groove (41) formed in the outer peripheral surface thereof; a nut member having a spiral-shaped load rolling groove so formed in the inner peripheral surface thereof as to correspond to spiral-shaped roller rolling groove (41), and a plurality of rollers (43) to be arranged and stored in a load rolling passage (44) formed between the roller rolling groove (41) and load rolling groove, wherein, as the screw shaft (42) is rotated with respect to the nut member, the nut member is allowed to reciprocate with respect to the screw shaft (42), characterized in that the number of forward-motion load rollers (43b) arranged such that the center line (46) of the rollers (43b) are set at a given angle (xcex4) with respect to the center line (47) of the screw shaft (42) and he number of backward-motion load rollers (43a) arranged such that the center lines (45) the rollers (43a) are so set as to cross the center lines (46) of the forward-motion load rollers (43b) are so set as to be different from each other according to the ratio of axial-direction loads applied to the roller screw during the reciprocating motion thereof.
According to the third aspect of the invention, in accordance with the ratio of the axial-direction loads applied to the roller screw during the reciprocating motion thereof, the number of the rollers (43) which receive a high load can be increased and the number of the rollers (43) which receives a low load can be decreased, which takes it possible to minimize the whole length of the nut member.
According to a fourth aspect of the invention, in a roller screw as set forth in any one of the first to third aspects of the invention, the nut member (14) includes return passages (20, 21) for connecting together one end and the other end of a load rolling passage (15) defined by the load rolling groove (13) and roller rolling grove (11), in order to allow the rollers (16) to circulate.
According to the fourth aspect of the invention, since the rollers (16) are alloyed to circulate, the stroke of the nut member (14) with respect to the screw shaft (12) can be increased.
Also, according to a fifth aspect of the invention, there is provided a roller screw roller arranging method for arranging a plurality of rollers (16) between a spiral-shaped roller rolling groove (11) formed in the cuter peripheral surface of a screw shaft (12) and a spiral-shaped load rolling groove (13) so formed in the inner peripheral surface of a nut member (14) as to correspond to the roller rolling groove (11),
wherein a forward-motion load roller group (17) is arranged in such a manner that the centers of the rollers (16) are set at a given angle (xcex1) with Respect to the center line of the screw shaft (12), a backward-motion load roller group is arranged in such a manner that the centers of the rollers (16) are set at an angle (xcex2), which is different from the above-mentioned given angle, with respect to the center line of the screw shaft 12), and the forward-motion load roller group (17) and backward-motion load roller group (18) are disposed in such a manner that they are separated from each other.
According to the fifth aspect of the invention, as described above, since the forward motion load roller group (17) receives the forward-motion axial direction load (1) occurring during the reciprocating motion of the roller screw and the backward-motion load roller group (18) receives the backward-motion axial direction load (2) occurring during the reciprocating motion of the roller screw, the roller screw is able to receive both of the forward- and back-ward-motion axial-direction external loads. Also, because the forward-motion load roller group (17) and backward-motion load roller group (18) are separated from each other, the two groups of rollers (16) are allowed to circulate individually, thereby eliminating a fear that the circulating motion of the rollers (16) can be interfered.